Positioning determining system



May 15, 1956 A. F. HASBROOK 2,746,034

POSITIONING DETEBMINING SYSTEM Filed June l, 1951 2 Sheets-Sheet l BY wam, @fg @umg y LL/@2,

ATT( )RNEKS May 15, 1956 A. F. HAsBRooK 2,746,034

POSITIONING DETERMINING SYSTEM Filed June l, 1951 2 Sheets-Sheet 2 fra/Ls'.

,c2 fare/rer 5 75; iz. fz' Y Fece/ver /z-y 5- INVENTOR ATTOR NE YS United States 2,746,034 Patented May 15, 1956 ice 2,746,034 PosITIoNlNG DETERMINING SYSTEM Arthur F. Hasbrook, BexarvCounty, Tex., assignor to Olive S. Petty, San Antonio, Tex.

This 'invent-ion relates to an improved method and apparatus for the electronic measurement of distances, whereby the location of `a remote position may be ydetermined. An import-ant feature of the invention is its applicability in locating positions in mountainous terrain and other regions which are diicult of access.

Various electronic systems Afor locating a remote position with reference to one or more known positions have been heretofore proposed. `In one general class of such systems, a master 'transmitting station located -at the unknown position is used in measuring distances to repeater stations at known positions. In another general class of such systems, `two master stations at each of two known positions are employed to measure the distances from each to a -repeater station Iat the unknown position. In certain applications, for instance the location of aircraft in space, such systems are fairly satisfactory, ybut when employed for other purposes, for instance surveying on Ithe earths surface, and especially in mountainous regions or other areas offering obstacles to transportation of heavy equipment, it is highly desirable to provide a system which is more readily adapted =to the terrain.

It 4is therefore an object -of the invention to provide a system of electronic surveying in which the master, or primary transmitting station, may be located at a known and readily accessible position, only simple repeating st-ations, employing readily portable equipment, being required at other points.

yMore specifically, it is an object of the invention to provide an electroni-c surveying system comprising a master station located at a known accessible posit-ion, `and repeater stations located at two remote points, of which the position of at least one is unknown, `the location of the other remote point being determinable -by conventional electronic or optical instruments. In the preferred operation lof the system, a sign-al is transmitted from the master station to the more -readily located point, from which it is reradiated for reception at the master station .as wel-l as at .the unknown point, being returned from the latter to the more readily located point and thence tothe master station. The returned signals, one series of which has been directly returned from Ithe iirst remote station and the other of which has traversed the additional distance between the two remote stations, are then utilized at the master station to measure `the distance between `the two remote stations, preferably by means affording a direct reading of the distance.

yThe preferred system also contemplates the transmission of signal energy from the remote station at the unknown location to the master station, thus affording a direct measure of -the 4distance between the last mentioned stations.

'Further objects and features of the invention will be apparent from the following description taken in connection with the -accompanying drawings, 'in which 4Figure -l is a geometrical illustra-tion of an arrangement of several stations, the, location of one of which is to be determined;

Figure 2 illustrates schematically a measuring system embodying the principles of the invention land indicating diagrammatically the equipment required at each of the several stations;

Figures 3 and 4 are wiring diagrams of certain components of the equipment at the base or master station;

Figure 5 is a graphical representation of signal energy appearing at various points of the equipment employed at the master station; and

Figure 6 is -a diagrammatic showing of a modification of part of the system illustrated in Figure 2.

In order to promote an understanding ofthe invention, reference is made to the specific embodiment thereof illustrated in the accompanying drawings and the same is described in detail hereinafter. It will nevertheless be understood that such modifications Iand alteration-s of fthe invention lare contemplated as would normally occur toy those skilled in the art to which the invention relates.

Figure 1 illus-trates a measuring system lof the character illustrated more fully in :the remaining figures, consisting of a master station located at M, a first remote or repeater station B1, and a second remote or repeater station B2, it being desired `to locate the position of station B2. The

three stations define a triangle, of which lthe sides d1, d2

and d3 represent the distances between the several stations.

In the presently described arrangement, stations M andk B1 are at xed loca-tions such that distance d1 remains constant; the direction of d1 relative to a known landmark may be determined by other means such as radio `or optical angle measuring instruments. Then it is only necessary to measure distances d2 and d3 for each location of B2 in order to determine the latters position. Because of the necessity for -a high degree of portability of 'the equipment to be used .at B1 and B2, all measure- Iments are made at master station M as follows: Distance d1 is measured by -transmi-tting signal energy from M to Bl and return and measuring elapsed time; distance d2 is similarly measured by transmission from M to B2 and return. Distance d3 i-s measured by utilizing all three stations; :the transmitted energy travels from M -to B1 to B2 and returns over the same cincuit; thus total distance dl-l-d is Itraversed and, a-s will be shown shortly, distance d3 may readily be obtained by effectively subtracting, in the electronic measuring circuits, the time required for traverse of d1.

Turning to Figure 2, in which the various components of the system are represented schematically, it will be observed that signal energy is generated in a device called for convenience a timer, which may be constituted by any conventional electronic circuit capable of generating timing signals,preferably in the form of signal` pulses of constant frequency. An improved circuit for the production of pulsed signal energy and satisfactory for use as a timer 10 in the .present system is illustrated and described in my prior application for U. S. Letters Patent, Serial No. 92,884, filed May 12, 1949, although it will be understood that the pulse generating means employed in my system may vary widely. In fact, l may employ, as described moreparticularly in my prior application for U. S. Letters Patent, Serial No. 84,359, tiled March 30, 1949, a sine wave transmission, pulses being formed only after transmission and reception of the signal energy, and then only for the purpose of greater accuracy in measuring. Whether pulses or marked sine waves are employed for transmission, the repetition rate or frequency is, of course, selected to suit the distances involved and the required degree of accuracy.

In any event, it is preferred that the transmission of signal energy be elfected by modulated carrier waves, and reference will be made herein, for convenience, to the modulating signal energy supplied by timer` 10 as pulsed rather than continuous wave form energy.V Con- 3 sequentlyl havesliow'nthe' output of timer 10 as supplied toa pulse modulator 30; which mayI be constitutedv by any conventional circuit whereby the pulsed energy delivered by timer is applied to the modulation of a carrier wave of constant frequency having an arbitrary value" F1. From' thepulse modulator the modulated carri'er' issupp'lied through' selector switch 13A to F1 transmitter 11, from which it is radiated to F1 receiver 20' at station B1. The equipment necessary at station B1 is m'ere'ly'that required to repeat and retransmitthe signals collected by* F1 receiver'20'. For reasons hereinafter eX- plained, it'A is desirable to' retransmit signals arriving at station B1 on a diierent carrier frequency, designated` F2', and F1' receiver 20l and F2 transmitter 21 at station B1 will, of course, include conventional means for demodulating the received signal and applying the resultant pulsed energy to the modulation of a carrier wave of frequency F2. This modulated wave is radiated to F2 receiver 25 at repeater station B2 and to F2 receiver station at master's'tation M, each of these receivers being tuned tothe carrier frequency F2' and being therefore unresponsive to signals received from F1 transmitter 11`. The carrier wave arriving at station B2 is demodulated, applied to a carrier wave of frequency F1, and' reradiated by F1 transmitter 2'6 to F1 receiver 20 at station B1 and' by F1 receiver 1'6 at master station M. A time` gate circuit 22, hereinafter more fully explained, is also employed at station B2 for suppressing or pre venting regenerative transmission of signals between stations B1 and B2, as hereinafter more fully explained.

The modulated carrier wave from pulse modulator 3i()V may', alternatively, be supplied through terminal 1 of selector' switch 13A to F2 transmitter 12 at master station M for radiation to F2 receiver 25 at station B2, whereby signal energy may be transmitted directly from master station M to' station B2 and there retransmitted on frequency F1 for reception by F1l receiver 16 at master station M.

The equipment at master station M also includes a suitable indicator 1'4 of conventional design which is supplied with a selected reference or ranging signal, whereby the time elapsing between transmission of signals from master station M and return of the signals from the repeaterstations may be accurately measured. I may employ, for instance, an indicating circuit ofy the type disclosed in' my cope'nding application for U. S. Letters Patent, Serial No. 68,593, tiledV December 2l, 194'8.

Alternatively, I may employ a cathode ray tube on which is displayed the reference or ranging signal and the re turned signal, an adjustable delay circuit', giving a direct indicationl of the distance traversed by the repeated sig nal, being'employed to bring the ranging and the returned pulses into alignment. The extent of the required adjustment of the delay circuit is thus a measure of lapsed time, and the adjusting means may be calibrated to give a direct reading in terms of distance. One such arrangement is described in my prior application for U'. S. Letters Patent, Serial No. 84,360, led March- 3l), 1949.

Thus, as illustrated herein, the ranging signal may be supplied through selector switch 13B from any one of the ranging circuits 17, 18 and 19, ranging circuits 17 and' 18 being energized directly from timer 10, and ranging circuit 19 being energized from F2l receiver 15 through suitable gating controls, hereinafter described. The signals received at master station M from the repeater stations by F2 receiver 15 and F1 receiver 16 are supplied to the indicator through selector switch 13C. As indicated, selector switches 13A, 13B- and 13C are coupled for concurrent operation. The mode of operation of the system as thus far described is as follows.

Assuming thatv it be desired rst to measure the distance d1 from master station M to repeater station B1, selector switches 13A, 13B and 13C are` set at position'3,

F1 transmitter 11, F2 receiver 15, and ranging circuit 17' being thereby rendered operative. The signal transmitted on F1 frequency from transmiter 11 is received at F1` receiver" 20" at St'atio'ri' B1, conver'td1 to" Carrierv frequency F2, reradiated by transmitter 21, and received by F2 receivers 25 at station B2 and 15 at master station M. At station B2, the received signals are applied to carrier frequency F1, reradiated by F1 transmitter 26, and received by F1 receiver 20 at station B1 for reradiation on frequency F2 by transmitter 21 for reception by F2 receiver 15 at master station' Thus the pulsed signal energy radiated from master station M to station B1 is there received andl repeated, being intercepted by receiver 15 at the master station and applied to indicator 14. The r-anging circuit'17'a'lso supplies-a suitable reference signal to indicator 14 which,by adjustment to coincidence with the pulse derived from the transmission and reradiation just described affords a direct reading of the distance d1 between master station M and station B1.

It will be appreciated thatv the signalV energy transmitted from master station M through station B1 to station B2, and back over the same path to master station M arrives at receiver 15 later than the signal directly returned from station B1, since the distance traversed by the former is twice the sum of d1 anddi.l The lapse of time required for transmission of this later arriving signal may now be measured by adjusting selector switches 13A, 13B, and 13C to position 2, whereby trans-Y mitter 11 and receiver 15 remain kconnected as before, but the reference signal is supplied through ranging` circuit 19, whichV is energized. by the earlier received signal which has traveled only the path between master station` M, repeater station B1,and return.- Thus if the time delay in ranging circuit 19 is adjusted until the ranging pulse is in coincidence with: the later ofthe tWo arrivingsignals, adirect reading of distance d3 will begiven, the elapsedtime required for travel of signal energl between stations M` and. B1 being, in eifect, eliminated by the energization ofranging circuit 19'by a signal which has traversedl that distance.

It will be appreciated that by adjusting selector switches 13A, 13B, andV 13Cto position l, F2 transmitter12will be energizedl and F1 receiver 16 and ringingcircuit 1'8l will be connected to the indicator,l whereby signal energy will be transmitted from master station M directly to station B2, and will be directly returned, thereby giving a reading of distance d2 between the two stations.- Asexplained before, this reading may be employed asv a check on the readings previously obtained, even thoughy the position of station B1 withreference to'master station M is known, or has been previously measuredI by optical instrument orotherwise.

In order to obtain a' direct readingA of distance d3 in thel manner hereinbefore described, it is essential, of course, that only the signals directly returned from1 station B1 to master station M be applied to the energizing of the ranging circuit 19, and to screen oreliminate from ranging circuit 19 the later arriving signals which have traversedl the total distance dl-l-id'. Again, itis desirable to utilize circuits for this purpose which arek capable of minimizing noise, so as to prevent' energiza# tion of ranging circuit 1-9I'byl random energy. To achieve these results, I prefer to employr a gatingl circuit 50-` and a gated pulse amplifier 51, these units being arranged in the' system as shown in Figure 2, and beingreff'ective to prevent actuation of ranging circuit 19 except during the proper intervals, sol asA to minimize'actuation thereof byenergy other than the pulses returned directly fromv station B1.

Thus'referring to Figure3; illustrating details' of a pre1 ferred embodiment, it will be noted that unit 50 employs tubes 61 and 62, these tubes being connected'ina circuit which maybe described as-a catliode'coupled scalefof-two' circuit for actuation in sequenceby'signals applied separately to the control grids thereof. ln other words,-t1b'es 61 and 62- are alternately conductingand noncondu'cting., as determined by the sequential application of signals' to their grids. The circuit constants are such that tube 62 is nonconducting immediately before F1 transmitter 11 is pulsed, while tube 61 is conducting. On the emission of a signal pulse by transmitter 11, the pulse is detected by suitable means, for instance rectifier 64, which may be arranged in a resonant circuit, and the derived pulse is applied across grid resistor 72 to render the grid of tube 62 more positive, thereby initiating conduction in that tube. The resultant voltage fall at the plate of tube 62 is coupled through capacitor 67 and resistor 68 to the grid of tube 61, so that the plate voltage of the latter rises. This voltage rise or positive pulse at the plate of tube 61 is then fed through capacitor 74 to apply a positive potential to the screen grid of tube 75 of gated pulse amplifier 51.

In addition to tube 75, gated amplifier 51 includes as circuit components, cathode resistor 79 and by-pass capacitor 80; screen divider resistors 76 and 77; and plate load resistor 78 and grid resistor 81. The cathode bias on the tube and the effective D. C. screen voltage across resistor 76 are so chosen that, in the non-gated condition, tube 75 will not pass signals applied on its control grid through capacitor 82. Thus signals cannot pass from F2 receiver 15 through capacitor 82 to tube 75 and thence through capacitor 83 to ranging circuit 19 until tube 75 is gated into operation by the signal from circuit 50. When, however, a positive gating signal, generated in circuit 50 in response to pulsing of F1 transmitter 11, is applied to the screen grid of tube 75, the latter becomes operative to pass signals from receiver 15. The first returned signal of appreciable magnitude arriving thereafter at receiver 15 will be the pulse representative of distance d1, and this signal is applied to the control grid of tube 75 to energize ranging circuit 19 through capacitor 83. At the instant circuit 19 is energized, the signal is also applied through capacitor 63 to tube 61. This initiates conduction in tube 61, resulting in a'voltage fall at its plate and terminating the gating pulse applied by circuit 50 to tube 75, whereby the latter is rendered inoperative. Simultaneously, this voltage fall at the plate of tube 61 is applied through capacitor 65 and resistor 66 to the grid of tube 62, returning the latter to its original state of nonconduction, and thus preparing it for actuation by the next succeeding pulse derived from F1 transmitter 11.

The sequence of operations just described is illustrated in graphic form in Figure 5. Thus, the cycle of operations begins, as shown in graph I, with the transmission of a pulse from F1 transmitter 11. At that instant gate 51 is rendered operative as shown in graph II, and on arrival of the first returned pulse at F2 receiver 15, ranging circuit 19 is energized. Immediately thereafter, gate 51 is rendered inoperative, and remains so until the next pulse is radiated from F1 transmitter 11. Consequently neither the arrival at F2 receiver 15 of the later pulse which has traveled the entire distance to station B2 and return, nor echoes which may follow the arrival of pulses, is able to actuate ranging circuit 19. As shown on graph I, echoes of appreciable magnitude are not received after transmission of a pulse from Fl transmitter 11, since F2 receiver 15 is not tuned to the frequency of the transmitter, and any direct effect from the transmitter is negligible.

In Figure 4 is shown a circuit suitable for use as a time gate at station B2, for the purpose of preventing regenerative transmission between stations B1 and B2, as hereinbefore mentioned. Thus vacuum tube 100, transformer 101 and the associated components comprise a blocking oscillator which performs the dual function of providing a driving pulse for transmitter 26 and operating as a time gate 22 to suppress regenerative pulsing between stations B1 and B2. Transformer 101 is so connected with tube 100 as to afford single pulse output when a trigger signal from receiver 25 is applied through coupling capacitor 104. The grid of tube 100 is normally biased below cut-off by a positive-voltage on the cathode impressed by the network 105and 107,

capacitor 106 serving to by-pass resistor 105 effectively;

Capacitor 108 and resistor coupling purposes. When a trigger pulse is impressed on input capacitor 104, regenerative action occurs, as described at length in my copending application for U, S. Letters Patent, Serial No. 92,884, filed May 12, V1949, and a single pulse output is furnished from transformer so as to excite the R. F. oscillator or amplifier in transmitter 26. Now the recovery time of the blocking oscillator, as illustrated in the waveform associated with the grid, may be adjusted by selection of 103 and 102 so as to form a time gate which effectively prevents received pulses from triggering the circuit except at the proper pulse rate. Thus the time of recovery may be arranged so that unwanted pulses, such as the retriggered pulse from B1 which occurs at time t, will not trigger the blocking oscillator. By time T, however, the `circuit has fully recovered so as to be responsive to the next proper trigger pulse at the proper pulse rate established by timer 10 in master station Other circuits may be employed for the purpose, but l prefer that shown in Figure 3 for its simplicity and effectiveness. For instance, under certain conditions of operation and especially where the received signal-to- 109 -are employed for denoise ratio is low, the circuit of Figure 6 is preferable."

In this figure, d3 ranging circuit 19 is triggered by th'e range pulse output from d1 ranging circuit 17,A thus automatically providing subtraction of d1 dbi-d3 received signal. The dial on d3 ranging circuit 19 then yields the value of distance d3 directly; It will be appreciated that the output of d1 rangingxcircuit 17 is free from noise since this circuit is not actuated by the F2 receiver 15 and therefore does not pick up the usual atmospheric interferences. It is first necessary to` set d1 ranging circuit 17 properly before d3 ranging circuit is adjusted, and in this respect the circuit of Figure 6 is not so simple to operate as that previously described, but the freedom from noise is under certain conditions,

It will be appreciated that the system described herein affords an effective method of measuring, by direct transmission and return of the signal, the distance between the master station and cach of the ltwo remote stations, and by transmission of the signal from the master station through the rst remote station to the second remote station and return, the distance between the two remote stations. As a check on these measurements, a signal may be transmitted over the fullperimeter of the triangle, the signal being transmitted from master station M to repeater station B1 to repeater station B2, from which it is directly returned to master station Mf,^and the total distance determined from the travel time. It will also be understood that the method of determining the distance between two remote stations described herein may be effectively employed in a system involving the use of additional repeater stations, for instance,a system employing three repeater stations and a master'station, so that the transmission paths define a quadrilateralfigure. In such a system, the procedure outlined herein may readily be employed tol determine the length of the two remote sides of the ligure, the two sides of the figure adjacent the master station being measured by direct transmission and retransmission.

Various other fields of use of the invention will be apparent to those skilled in the art, for instance the de,- termination of the instantaneous position of remote moving stations, such as aircraft, ships and land vehicles. All such useful aspects and applications of the inventive principle illustrated herein are contemplated as part of the invention defined more particularly hereinafter.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A system for measuring at a base point the distance from the distinctly advantageous between titi/( r'rb'te pointe' Comprising', in c'0`rninitiation",

a master' station located atisaid basepoint a'nda"r`e`pea`ter stationl lcated a'tt ea'chi of said remote' points;- each' sta'- tioh inclLid-ing a1A transmitter and: a receiver, means at eachrof; said? repeater'pstatnns connecting the transmitterand receiverr toi effect reradfation by the transmitter of signals" arr'ivingl at the receiver; the' transmitter at said master station,A th'ereceiver at the first of saidl repeater stations, and thev transmitter at the second of said repeater' stationsI bein'gtuned to a first' frequency; the rece'iverl at said master station, the transmitter at the first ofi said'f repeater stations' and the receiver at: the second ofsaid'repeater stations being tuned to a second frequency;

wherebysignals transmitted from said masterv stationl are initially receivedv only at the! first ofv said repeater stations;- signalsfrer'adiate'd from thesecond of said repeater Sta'tiIlS a'le initially-'received' Only at' the i'r'stkk 0f Said repeaterstations; andi signals reradiated from thev first of said repeater stationsY are" received at both said master station and the secondf of"4 said repeater stations; so that signals transmitted front: said master station" are returned directly to said master station from the'first of said repeater stations, andl are concurrently reradiated to the second of said` repeater stations', returned to the first of.' saidv repeater stations and thence to said master' station; andI means at said master' station for measuring the lapsedtime'between the arrival atv said master station off signals returned directly from' the first of said repeater stations andl of signals returned from the second of'l saidrepeater stations through' the first of said repeaterfstations to said masterstation.

2. A system'f'or measuring at abase pont the distance between' twov remote points` comprising,y in combinationy a-ima'ster station'located at said base point and a repeater stationilocated at'each of said remote points; each station-including a transmitter and a'receiver; means at each of said repeater stations connecting the transmitter and receiver'toteffect'reradiation by the transmitter of signals arrivingat the receiver; means for receiving-signals from saidl master stationl only at the first of said repeater stations, means receiving at the first of said-repeater stations onlyv signals' reradiated from the' second of saidr repeater stations;l and meansreceiving at bothy said master station andthe second-of said repeater station signals reradiated from'- the first;v of said repeater stations; so that signals transmitted-from said'maste'r station are returneddirectly to said-master stationfrom the first' of'said repeater sta tions, and are concurrently reradiated to the secondof to the first of saidY resaid master station; and

said repeater stations, returned peater stations andthence to means at saidmaster station for measuring the lapsedv time-'between the'initial arrival at said master station of' signals'- returned directly from the first ofV said repeater stations and the later arrivalof signals'returned from) the second of said repeater stations through the first of said repeater stations to said master station.

3. A system for measuring at a base pointl the distance between two remote points comprising, in combination, a master station located at said base point and a repeater station locatedV atv each of said remotey points;

eachstation includinga transmitterl and a receiver; means at each of said repeater stations connecting the transmitter and receiver to effect reradiation by the transmitter ofsignals arriving at the receiver; means for receiving signals from said master station only at the first of` saidA repeater stations, means receivingat the first ofsaid repeater stations only signals reradiated from the second ofsaid'repeater stations, and means receiving at both saidmaster station and the second of said repeater station signals reradiatedfrom` the'first of said repeater stations; so that signals transmitted'from saidmaster station are returned directly. to saidv master station from the first of said repeater stations, and are concurrentlyfr reradiated to' the' second of saidV repeater stations, returned to the first of-said' repeater stations and thence to said master station; and-means ati said master. station for measuring the lapsedv time between'A the' initial arrival at said? masterr stationlo'f signals' returned directly' frm the first-of said repeater stations andA the later arrival'of signalsre'turried from' the` second of' said repeater stations through the first of said repeater stations to said master station, said last named means comprising an' indicator, devices r'e-' sponsive to signal energy returned directly fromthe first of said repeater stations for supplying a reference pul'se` tion thereof in response to signal energy returned from the second of said repeater stations. l

4. A system for measuring at a base point the distance between two remote points' comprising', in combination,-

a master station located atsaid base point and a repeater station located'at each of saidremote points; each station including al transmitter'and a receiver; means at each` of said repeater stations connecting the transmitter and receiverv to effect reradiation by the transmitter of signals arriving atl the receiver; means for receiving signals from saidy master station only at the first of said repeater sta'- tions, means receiving at the rst of said repeater stations only signals reradiatedfrom the second of said repeater stations, and means receiving at both said master station and the second of said repeater station signals'reradiated from the first of said repeater stations; so that signals transmitted from said master station are returned directly to said master station from the first of said repeater stations, and are concurrently reradiated to the second of said repeater stations, returned to the first of said repeater stations and thence to said master station;

and means at said master station for measuring the lapsed time between thefinitial arrival at said master station of signals returned directly from the first of said repeater stations and thelater arrival of signals returned from the second of said repeater stations through the first of said repeater stations to saidv master station, and devicesA at the second of said repeater stations for preventing reradiation by its transmitter of signals previously'receivedl and reradiated from said stations, whereby regenerative action between said repeater stations is minimized.

5. A system for measuring at a base point the distance between two remote points comprising, in combination, a master station located at said base point and a repeater station located at each of said remote points; each station including a transmitter and a receiver, means at each of said repeater stations connecting the transmitter and receiver to effect reradiation by the transmitter of signals arriving at the receiver; the transmitter at said master station, the receiver at the first of said repeater stations, and the transmitter at the second of said repeater stations being tuned to a first frequency; the receiver at said master station, the transmitter at the first of said repeater stations and the receiver at the second' of saidirepeater stations being tuned to a second frequency; whereby signals 'transmitted from said master station are initially received only at the first of said repeater stations, signals reradiated from the second of said repeater stations are initially received only at the first of said repeater stations, and signals reradiated frommeasuring the lapsed time between the arrival at said master station of signals returned directly from the first of said repeater stations and of signals returned fromy the second of said repeater stations through the first-of said repeater stations to said master station, a second transmitter and a second receiverat said master station, said first and second transmitters being selectively and alternately operable, said second transmitter operating on said second frequency and said second receiver being tuned to said iirst frequency, whereby signals radiated by said second transmitter are received and returned directly to said master station by the second of said repeater stations for measurement of the distance therebetween.

6. A system for measuring at a base point the distance between two remote points comprising, in combination, a master station located at said base point and a repeater station located at each of said remote points; each station including a transmitter and a receiver, means at each of said repeater stations connecting the transmitter and receiver to eiect reradiation by the transmitter of signals arriving at the receiver, the transmitter at said master station, the receiver at the iirst of said repeater stations, and the transmitter at the second of said repeater stations being tuned to a first frequency; the receiver at said master station, the transmitter at the rst of said repeater stations and the receiver at the second of said repeater stations being tuned to a second frequency; whereby signals transmitted from said master station are initially received only at the first of said repeater stations, signals reradiated from the second of said repeater stations are initially received only at the rst of said repeater stations, and signals reradiated from the first of said repeater stations are received at both said master station and the second of said repeater stations; so that signals 10 transmitted from said master station are returned directly to said master station from the rst of said repeater stations, and are concurrently reradiated to the second of said repeater stations, returned to the iirst of said repeater stations and thence to said master station; and means at said master station for measuring the periods of elapsed time between transmission of signals from the master station, the arrival at said master station of signals returned directly from the first of said repeater stations, and the arrival of signals returned from the second of said repeater stations through the first of said repeater stations to said master station, said last named means comprising an indicator, and devices supplying to said indicator' signal pulses indicative of the time of (1) signal transmission from said master station, (2) arrival at said master station of directly returned signals, and (3) arrival at said master station of signals returned from the second of said repeater stations.

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